Stability of the MIDI photometry: the case of Circinus

In principle, the MID-infrared Interferometric instrument (MIDI) at the Very Large Telescope Array (VLTI) should always measure the same calibrated total flux spectrum for a specific source, independent of the instrument settings and the baseline geometry. In the data on the Circinus galaxy, however, there is (a) a general offset of the flux values for 2009 and (b) a slow drift of the total fluxes at short wavelengths during two nights (2008-04-17 and 2009-04-14). The latter seems to depend on the hour angle of the observation. In this document, a more detailed analysis of these two effects is carried out and summarised. The goal is to find an explanation for these variations in the photometry.